Which of the listed examples is a current hypothesis regarding the Cambrian explosion?

• A. An increase in atmospheric CO₂ led to an explosion of plants and life-forms that fed on plants.
• B. Increased numbers of predator species led to a dramatic decline in prey species.
• C. A rise in atmospheric nitrogen led to success of large animals with higher metabolic rates.
• D. All of the listed responses are correct.
• E. Evolution of the Hox gene complex provided developmental flexibility.